SEMINAR ON APPLICATIONS OF VAPOR

GEOCHEMISTRY

CONTENTS INTRODUCTION SOURCE OF NATURALLY OCCURING GASES MOVEMENT OF GASES EXPLORATION FOR SULFIDE DEPOSITS EXPLORATION FOR URANIUM EXPLORATION FOR PETROLEUM LOCATION FOR BURIED FAULTS FORECASTING OF EARTHQUAKES FORECASTING OF VOLCANIC ERUPTIONS LOCATION OF GEOTHERMAL AREAS VAPOR SURVEY TECHNIQUES CONCLUSION REFERENCE

INTRODUCTION• The gases emanating from ore deposits or oil fields

have recently attracted increased attention as possible exploration guides.

• Very little development as indicators of various geological phenomena.

• The gases which can be studied in three main environments: open atmosphere, pore space of soil and over burden, surface and ground waters with dissolved gases.

SOURCE OF NATURALLY OCCURING GASESa. ATMOSPHERIC GASES•O2 is the dominant reactive constituent. Surface water with dissolved O2 circulates to depths of tens or hundreds of meters below surface to set up an oxidizing environment.•Atmospheric CO2 maintained at 0.035% by equilibrium with CO2 dissolved in sea water.

b.

b. DEEP-SEATED GASES•Deep seated environment characterized by low oxygen fugacity (effective concentration of elemental oxygen) and relatively high fugacity of reducing gases, such as H2 and CH4.c. RADIOGENIC GASES•Deep-seated or shallow origin•Trapped in parent rock. Eg. He, Rn, K.

Cont…d. BIOGENIC GASES•Disintegration of organic matter•eg. CH4, H2, H2S, CO and H20

e. GASES GENERATED IN SULFIDE DEPOSITS•Abundance of free O2 in buried sulfide deposits undergoing oxidation at grater depth.•The sulfur in sulfides react with H20 and 02 to form sulfuric acid.

f. Atmospheric Particulates•Water droplets in the form of clouds and fog, are the most common particulate materials in atmosphere.

MOVEMENT OF GASES

a. DIFFUSION•Diffusion may be defined as the flow of a species from a region of high concentration to low concen- tration as a result of random thermal motion.•Diffusion through a viscous liquid will be slower than in open, unrestricted systems.b. WATER TRANSPORT•Mass movement of aqueous solutions of volatile species through rocks and open fissures can be several orders of magnitude faster than movements by diffusion.

Cont…c. TRANSFER BETWEEN WATER AND VAPOR PHASES•Evaporation and condensation of gases are continually taking place at any aqueous phase and a vapor phase.•For gases generated at depth, a net upward movement of gas from the water to the overlying vapor occur be-cause the gas is continually replenished in water and leak away towards the atmosphere.d. VAPOR TRANSPORT•Except in volcanic systems, mass movement of vapor is restricted to the aerated zone above the water table.

Cont….Only factors that can have a appreciable effect on the flow of air in aeration zone are changes in pressure, temperature and moisture, which are related either directly or indirectly to changes in weather.

e. RELATIVE EFFECTS OF INFILTRATION VERSUS DIFFUSION•Diffusion is certainly the principal factor in the movement of gases through impermeable rocks.•Transport of volatile species by infiltration of aqueous solution must dominate in aquifers both in permeable rocks and in fractured igneous and metamorphic rocks.

EXPL0RATION FOR SULFIDE DEPOSITS• Free or dissolved gas associated with oxidizing

sulfides ores report strong anomalies in CO2. • Anomalies in soil air reported in these investigations

fall in the range 1.5-4% v/v, the normal contents usually fall well below 1%.

• The CO2 content if gases dissolved in ground water near oxidizing ore may be as high as 10% compared to a background of 0.2%.

• Increases in CO2 are not uncommonly paralleled by decreases in O2.

Exploration For Uranium• The use of Rn in soil gas and in natural waters in

locating buried U deposits is becoming well established as a prospecting method(smith et al., 1976).

• In several areas, Rn anomalies have been detected above ores covered by up to100 m of transported overburden.

• In other circumstances , strong control of Rn anomalies by faults or other permeable zones, or by U dispersed away from its primary source, has been demonstrated (Michie et al ., 1973).

EXPLORATION FOR PETROLEUM• Light hydrocarbons and the radiogenic gases He and Rn. • Methane, the dominant member of the hydrocarbon

group, tends to be most concentrated in the highest part of the oil structure, where it may accumulate as a separate vapor phase in the form of natural gas.

• The radiogenic gases are generated by U which tends to be precipitated at the water-oil interface at the base of the oil accumulation.

• In theory, He and CH4 can diffuse directly upward through the impermeable cap rocks to form projected images near the surface that reflect their distribution at depth.

LOCATION OF BURIED FAULTS• Russian workers have carried out extensive studies of He

in soils and sedimentary cover as a guide to deep faults and basement fractures(Bulashevich,1974; Eremeev et al ., 1974; Ivanov and Medovyi, 1975; Ovchinnikov et al .,1973).

• Mercury has also shown promise as a guide to deep fracture zones. Carbon dioxide and other gases shown anomalies along near surfaces faults and fracture zones (Fridman and Petrov, 1976; Rosler et al ., 1977).

FORECASTING OF EARTH QUAKES• Recent studies have suggested that immediately prior to

an earthquake, pervasive crackling and dilatancy occurs throughout the rocks adjoining the potential fault surface, resulting in the release of certain occluded vapor species.

• These gases are then free to move toward the surface where they may be measured as components of the soil air.

• The only vapor indicators of forthcoming earthquakes so far reported are the radiogenic gases Rn, Ar, and He.

FORECASTING OF VOLCANIC ERUPTIONS

•Stoiber and Rose (1974) found that the fumarolic condensates, leachates, and encrustations decreases before a volcanic eruption, stays constant during the eruption, and then returns to its normal value afterwards.

• Chirkov (1976) has reported that the Rn content of volcanic fumaroles and hot springs in the U.S.S.R. increased before and during the eruption of a volcano.

LOCATION OF GEOTHERMAL AREAS

Certain gases in soil air, particularly He, CO2, and Hg, may under favorable conditions indicate the presence of thermal water at depth (Koga and Noda, 1975; Reimer et al., 1976; Tikhomirov and Tikhomirova, 1971; Hinkle,(1978). These constituents are apparently released by the heat and active water circulation of the thermal zone.

VAPOR SURVEY TECHNIQUES The particular advantage of vapor as an indicator of geological

phenomena that are hidden either in the ground or in the future lies in its high mobility. This same quality also makes vapors probably the most difficult of all geochemical indicators to sample, analyze, and interpret.

If the geochemical signatures that are being carried upward by vapors from the depths are to be deciphered at all, the techniques of sampling and analysis must be perfected so that even very low concentrations can be reproduced.

More research and trial surveys are also needed to understand the natural processes whereby gases are transported and destroyed, and the limitations of vapor surveys.

Cont….Variations in pressure, temperature, and moisture strongly affect the mobility and hence the relative concentration of the various gas species in soil air. Well defined anomalies in soil air have been known to vanish entirely on resampling. Sampling soil air directly, therefore, can be an extremely frustrating experience. Fortunately, however, soil air is not the only medium that will give us a measure of the gases escaping from depth. The gases dissolved in ground water or sorbed to natural traps or artificial traps, though less direct, may give values that tend to level out the time variations that are so dependent on the weather.

CONCLUSION• Active oxidation of the sulphide mineralization

appears to be an essential condition for the generation and dispersion of gases.

• Collection of three consecutive soil-gas sub samples from boreholes 2 to 4 m deep provides sufficient information for prospecting purposes.

• The nature and age of the overburden materials as well as the supergene history of the deposits studied are important in the interpretation of vapor geochemical prospecting data.

REFERENCE

• Geochemistry in mineral exploration Rose,A.W Hawkes. H.E & webb J.S 1979, pp-489-511.

• Rock geochemistry in mineral exploration. G.J.S.Govett.Elsevier publication.1983, pp-123-135.

• www.Wikipedia.com• www.google.com

THANK YOU